Theoretical study on the kinetics of hydrogen cyanide and hydrogen isocyanide reactions with the methyl radical

The detailed kinetic mechanisms for the reactions of hydrogen cyanide (HCN) and hydrogen isocyanide (HNC) with the methyl radical (CH₃) are discussed. These are important reactions in combustion and Titan's atmosphere chemistry and were investigated at the CCSD(T)/cc-pVQZ//M06-2X/6-311++G(2df,2p) level of theory. The multiwell and multichannel potential energy surface (PES) was constructed. The rate constants were determined by using variational transition state theory (VTST) and Master Equation/Rice-Ramsperger-Kassell-Marcus (ME/RRKM) method over a temperature range of 300-2000 K and a pressure range of 1-10 000 torr. Corrections of the Eckart tunneling effect were included and the calculated results were in good agreement with the literature. A clear dependence of the reaction mechanism on temperature and pressure was revealed via detailed kinetic and species analysis. For the HCN reaction, the channel of C-addition forms an intermediate that is dominant at low temperatures and high pressures, leading to the total rate constant exhibiting a pressure dependence, but this dependence disappears at high temperatures. The H-abstraction channel is more competitive with increasing temperatures, but it is still not dominant. For the HNC reaction, the C-addition channel is dominant, and CH₃CN and H constitute almost all the products. The proposed temperature and pressure-dependent rate constants can be used in the combustion and atmospheric model development for related systems.